Trends in wild adult steelhead (Oncorhynchus mykiss) abundance for snowmelt-driven watersheds of British Columbia in relation to freshwater discharge

Abstract

Snowmelt-driven rivers of British Columbia support primarily summer-run steelhead (Oncorhynchus mykiss) that may spend up to 5 years as juveniles in freshwater. Time series analyses revealed significant negative correlations between an annual index of wild adult steelhead abundance (catch-per-angler-day, CpAD) for these rivers and summer freshwater discharge when these steelhead were juveniles. The strength of these relationships was related to latitude, with the more northerly rivers generating the strongest relationship between CpAD and freshwater discharge. Potential mechanisms by which interannual variation in freshwater discharge can modulate adult steelhead abundance include reduced juvenile mortality due to lower flow velocities during the warm summer months and to the creation of more juvenile habitat in low-velocity refuges. Alternatively, interannual variability in adult steelhead abundance is driven by variability in ocean climate of which freshwater discharge is an index. Interpretation of the data and analyses was encumbered in part by particular factors affecting CpAD as an index of abundance. However, the analyses support an interpretation that steelhead survival to adulthood might be influenced by freshwater conditions more so in northern snowmelt-driven rivers than in rainfall-driven rivers because steelhead from those rivers spend more years in freshwater as juveniles.